Hydraulic actuator

ABSTRACT

A hydraulic actuator is constructed in an enclosed housing having an internal wall member which divides its interior into forward and rear chambers. An elongated actuator element is slidably supported in the forward chamber with its forward nose portion extending outwardly through an opening in the wall of the housing. A piston is transversely supported on the rear end portion of the actuator element and is in slidable sealing engagement with a cylindrical wall contained within the forward chamber to define an expandable chamber with this cylindrical wall and the internal wall member. A spring means constantly urges the piston towards the internal wall member. Pump means contained in the rear chamber introduces a fluid under pressure through a passageway formed through the internal wall member into the expandable chamber to force the piston and associated actuator element outwardly of the housing to an extended position. The relative location of the actuator element is detected by a position sensing means which provides a variable control signal in response to the movement of the actuator element for energizing the pump means.

United States Patent [191 Nolden [111 3,828,556 [451 Aug. 13, 1974llYDRAULIC ACTUATOR [75] Inventor: William F. Nolden, Dallas, Tex.

[73] Assignee: Johnson Service Company,

Milwaukee, Wis.

[22] Filed: Jan. 26, 1973 [21] Appl. No.: 326,855

[52] US. Cl. 60/432 [51] Int. Cl. ..F15b 11/12, F15b 15/18 [58] Field ofSearch 60/432; 318/657; 137/487.5

[56] References Cited UNITED STATES PATENTS 3,108,213 10/1963 Golder eta1 318/657 3,200,591 8/1965 Ray 60/432 3,225,782 12/1965 Begley et a1.137/4875 3,626,283 12/1971 James 318/657 X OTHER PUBLICATIONS Gorman,Robert; Why Does a Pump Pump? Popular Science, Aug. 1962, pg. 107.

Primary Examiner-Edgar W. Geoghegan Assistant Examiner-William F. WoodsAttorney, Agent, or Firm.lohnson, Dienner, Emrich,

Verbeck 8Z Wagner 40 412 60 6 J2 JOJOJZ #72 [57] ABSTRACT A hydraulicactuator is constructed in an enclosed housing having an internal wallmember which divides its interior into forward and rear chambers. Anelongated actuator element is slidably supported in the forward chamberwith its forward nose portion extending outwardly through an opening inthe wall of the housing. A piston is transversely supported on the rearend portion of the actuator element and is in slidable sealingengagement with a cylindrical wall contained within the forward chamberto define an expandable chamber with this cylindrical wall and theinternal wall member. A spring means constantly urges the piston towardsthe internal wall member. Pump means contained in the rear chamberintroduces a fluid under pressure through a passageway formed throughthe internal wall member into the expandable chamber to force the pistonand associated actuator element outwardly of the housing to an extendedposition. The relative location of the actuator element is detected by aposition sensing means which provides a variable control signal inresponse to the movement of the actuator element for energizing the pumpmeans.

16 Claims, 10 Drawing Figures PATENTED AUG 1 31974 SHEU 2 BF 6 PAIENTEDwe 1 31974 SHEEI 6 BF 6 m 02 u 562E V I id ktsiom OZ mwwwzt. .ELEIOwwomnow o HYDRAULIC ACTUATOR BACKGROUND OF THE INVENTION This inventionrelates to hydraulic actuators, and more particularly to hydraulicactuators for longitudinally positioning a shaft.

A primary object of this invention is to provide a hydraulic actuatorhaving a self-contained, closed-circuit hydraulic system which isdesigned to provide greater strokes and greater power at extremely lowoperating cost.

Another object of this invention is to provide a hydraulic actuator forlongitudinally positioning a shaft having a pump-motor assembly thatoperates only on signal demand to substantially reduce the wear of thepump parts and power consumption.

These and other objects of the invention will become apparent withreference to the following description of the preferred embodiment anddrawings.

SUMMARY OF THE INVENTION A hydraulic actuator embodying the principlesof my invention comprises an enclosed housing having first and secondchambers separated by a dividing wall member. An elongated actuatorelement is slidably supported through an opening in the actuator housingand carries a piston which is in slidable sealing engagement with acylindrical wall of one of the first chamber's. A pump means disposed inthe rear chamber when energized introduces fluid 'under pressure througha passageway formed in the dividing wall member into an expandablechamber defined by the piston and cylindrical wall to push the pistonand associated actuator element outwardly against the biasing force of acompression spring. A dump valve means is provided for selectivelybleeding the fluid from the expandable chamber to permit the actuatorelement and piston to be retracted. A piston sensing means detects theposition of the actuator element as it slides, and provides a variablecontrol signal in response to this sliding movement to selectivelyenergize the pump means and the dump valve means.

DESCRIPTION OF DRAWINGS For a better understanding of this invention,reference may be made to the accompanying drawings, in which:

FIG. 1 is a cross sectional view taken along the axis of housing of ahydraulic actuator embodying the principles of my invention;

FIG. 2 is an exploded view of the component parts which when assembledprovide the dump valve assembly which is used in the hydraulic actuatorof FIG. 1;

FIG. 6 is a side elevational view of a modified actuator element withthe associated piston and transformer coil shown in dotted lines;

FIG. 7 is a front perspective view of the actuator element of FIG. 1;

FIG. 8 is a perspective view of the actuator element of FIG. 1 showingit being used to produce a vertical reciprocating movement;

FIG. 9 is a front perspective view of an actuator element embodying theprinciples of my invention being used to simultaneously operate aradiator valve and a damper element; and

FIG. 10 is an electrical schematic diagram illustrated in block diagramform for an electronic control system to operate the hydraulic actuatorof FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT There is illustrated in FIG. 1 across sectional view of a hydraulic actuator embodying the principles ofmy invention and generally designated by the reference numeral 20. Thehydraulic actuator 20 is constructed to be assembled in an elongatedhousing 22 having rear and forward fluid-tight chambers 24 and 26, andhaving an electronic component compartment 28 located above the forwardliquid-tight compartment 26. A narrow passageway 29 extendingsubstantially the entire length of the forward chamber 26 interconnectsthe front and rear chambers.

The elongated housing 22 has a generally rectangular shaped body portion30 with an internal wall 32 separating the front and rear chambers andis preferably made as a one-piece die-casting. The rear end of the bodymember 30 is closed by rear cover plate 34 which is attached thereto bya plurality of mounting bolts 36 and sealed by use of diaphragm 31.Likewise, the front end of the body 30 is covered by front cover plate38. A gasket 40 is sandwiched between the front cover 38 and the bodyportion 30 to provide a sealed connection.

An actuator assembly 42 is contained within the forward chamber 26 andcomprises a control element 44 slidably mounted axially through frontchamber 26 and extending forwardly of the housing 22. The control element 44 is constructed as a two-piece shaft in which the forward portion46 is made of a non-magnetic permeable material and the rearward portion48 is made of a magnetic permeable material. A threaded shank49extending from the forward end of rearward portion 48 is screwed into aconforming threaded aperture 51 formed in the rear end of forwardportion 46. A piston 50 is secured crosswise on the rear end of controlelement 44. Piston 50 isadapted to reciprocate in the bore of acylindrical piston cylinder 52, which is mounted at the rear end offorward chamber 26. A Quad-ring 54 provides a sliding sealingrelationship between the piston 50 and the bore of cylinder 52. Thepiston cylinder 52, piston 50 and internal wall 32 define an expandablechamber 53. The forward end of the control element 44 is adapted toslide through a bearing 56 which has a seal and is seated in theenlarged diameter stepped opening 58 of front cover plate 38.'Thebearing 56 is mounted in place by means of a plurality of mounting bolts59.

The intermediate length of control element 44 moves through the centerof a feedback coil assembly 60 as the control element 44 is extended andretracted, and the insertion of the magnetic permeable portion 48 ofcontrol element 44 varies the inductance of the feedback transformercoil assembly to provide a variable control signal as will be describedhereinafter. The electrical leads 66 for feedback transformer coilassembly 60 pass through passageway 29 and are fed into electroniccomponent compartment 28 through grommet 64. A compression spring 62urges the piston 50 and associated control element 44 to their retractedposition as illustrated in FIG. 1.

Outward displacement of control element 44 is effected by theenergization of motor-pump assembly 68 which comprises a motor unit 70and pump unit 72. The motor unit 70 has an upstanding core member 74(FIG. 1) supporting an offset field coil 76 at its upper end and havingan armature 78 extending transversely in its lower end. The motor 70 ismounted on a pair of spaced bosses 80 which extend horizontally from theinternal side wall 81 of internal wall member 32. A pair of mountingbolts 82 are used to fasten the motor on the spaced bosses 80 which havethreaded bores formed inwardly of their outer ends.

The pump unit 72 has a housing 86 constructed by stacking threesquare-shaped plates together in side-byside relation. The middle plate88, as depicted in FIG. 4, is formed with a Cloverleaf-shaped recess 90in its middle section, in which the upper and lower arcuate portions 92,93 are larger than the two lateral arcuate portions 94, 95. A pair ofcooperating pump gears 96, 97 fit within the upper and lower arcuateportions in intermediate relation. The three plates are aligned by apair of alignment pins 98, 99 extending through aligned openings in thethree plates. The three plates are held together and mounted on theinternal sidewall 81 by means of four mounting bolts 100 insertedthrough the four aligned corner apertures 102 provided in the threeplates and screwed into threaded openings formed in the internal wall32. The gear 97 is rotatably supported by means of an idle shaft 106whose ends are supported by the outer plate 108 and inner plate 109. Thegear 96 is keyed to a drive shaft 110 which is also supported by theplates 108, 109, and which extends outwardly of the outerplate 108 whereits outer end is coupled by means of a clevis pin 112 to the bifurcatedouter end of motor drive shaft 114. An opening provided in the innerplate 109, in alignment with arcuate recess 94 of middle plate 88,provides the inlet 116 (FIG. 4) for the pump unit 72. Similarly, anopening in the inner plate 109 in alignment with arcuate recess 95 ofintermediate plate 88 provides the outlet for pump unit 72. When thepump unit 72 is mounted on the inner wall 81, the inlet 116 for pump 72aligns with a pump inlet passageway 120 formed within internal wallmember 32 of housing 22 which is in communication with the rear chamber24 and has a filter screen 122 at its entrance to prevent foreignparticles from entering the pump unit 72. Similarly, the pump outlet 118aligns with a pump outlet passageway 124 formed through the internalwall member 32 and in communication with the forward chamber 26.

Mounted within pump outlet passageway 124 is a check valve unit 126 forpreventing the return of the fluid through passageway 124. Check valve126 comprises a fitting 128 having a diameter slightly smaller than thediameter of outlet passageway 124 with an O- ring 130 disposed about itsouter perimeter for providing a sealing relationship therebetween and'aspring loaded ball valve 132 for sealing off its inlet port 134.

Ball valve 132 is made up of a ball 133 and a cylindrical ball guide 135which are biased towards inlet port 134 by a compression spring 137which acts between a retainer'washer 139 and end collar 141 ofcylindrical ball guide 135.

The forward and rear chambers 24, 26 are completely filled with ahydraulic fluid and when the motor 70 is energized by a control signalthe pump unit 72 forces the fluid under pressure through the pump outletpassageway 124 into the area behind piston 50 to drive the piston 50 andassociated actuator rod 44 outwardly against the force of compressionspring 62. To accommodate the change in volume of forward chamber 26caused by the outward displacement of actuator rod 44, a vent 138(FIG. 1) is formed through the rear end plate 34 to permit the middleportion of diaphragm 31 to move inwardly as the control rod 44 isdisplaced outwardly.

As will be described in full detail hereinafter, the feedback coilassembly 60 provides a control signal to deenergize motor 70 in responseto the movement of the magnetic permeable portion 48 of control rod 44through its center. The linearity characteristics of the feedbackcontrol signal is improved by adding a magnetic disk 61 on the forwardface of piston 50 which is in a coaxial relation with control rod 44.

When the motor 70 is deenergized, the control rod 44 will be held in astationary position since the check valve 126prevents the fluid fromflowing back through the pump outlet opening 124. This manner ofoperation is a substantial improvement over prior art structures whichrequire the motor pump assembly to be continuously operated when holdingthe actuator rod in a fixed extended position.

Because ambient temperaturechanges cause the end of the control rodstroke to be variable, limit switch means 63 is provided to remove powerto motor 70 as the control rod 44 reaches the end of its extendedstroke. Limit switch means 63 comprises a pair of switch contacts 64, 64mounted on the rearward end of feedback coil assembly 60 ondiametrically opposite sides of control rod 44. A printed circuit boardwasher 65 carried on the forward end face of magnetic disk 61 provides ashort circuit path between the switch contacts 64, 64' whenever thecontrol rod 44 reaches the extended stroke position. The switch contactswhen short circuited provide a control signal to switch device 206 (FIG.10) for deenergizing motor 70 with the result that a constant stroke isattained regardless of ambient temperature changes.

To permit the control rod 44 to thereafter be returned to its retractedposition, dump valve assembly is provided, which when deenergizedprovides a return passageway through the internal wall 32. Referring toFIGS. 1, 2 and 5, the dump valve assembly 140 is assembled in acup-shaped housing 142 which has a central opening 144 in its end plate146. A doughnutshaped gasket 148 is abutted against the end plate 146when mounted on the internal side wall 81 in alignment with inletopening 150 is depicted in FIG. 1. A field coil 152 is seated within thecup-shaped housing 142 with tab 154 of its end flange 156 received inslot 158 at the outer end of cup-shaped housing 142. A cylindrical core160 is inserted through the center of coil assembly 152 and its threadedend portion 162 is screwed shaped housing 142. A valve fitting 164 issecured within the central opening 166 of threaded shank portion 162. Aring retainer 170 is frictionally urged over the outer end of core 160and engages the outer end flange 156 of field coil 152 (FIG. 5).

The actuator means for operating dump valve assembly 140 is provided bythe combination of plunger 174, armature 176, and armature retainer 178.The plunger 174 is slidingly mounted in the center axial opening 180 ofcore 160 and its threaded outer shank portion 182 extends through bothcircular aperture 184 in armature 176 and mounting aperture 185 at thecenter of retainer armature 178. A nut 186 is screwed on the end of thethreaded shank portion 182 to provide a unitary unit. The retainerarmature 178 has a pair of forwardly projecting arm portions 188 havinginturned fingers 190 which are inserted into a pair of diametricallyopposite circular openings 192 in the peripheral side wall of cup-shapedhousing 142. The movement of the fingers 190 within the pair of openings192 limits the extent of travel of plunger 174. A ball 196 is insertedin the space between the inner end of plunger 174 and valve fitting 164to provide a check valve which closes off port 198 upon armature 176being pulled inwardly by the energized field windings of coil assembly152.

Upon the deenergization of field coil winding 152, the attraction ofarmature 176 is terminated and fluid under pressure may pass through theinlet port 198, radially outwardly through radial openings 177 of core160 and pass into rear chamber 24 through a plurality of openings 199formed about the cup-shaped side wall area and through the end spacingbetween the armature and cup-shaped housing.

The threaded shank 162 of core 160 is used to mount the dump valveassembly 140 in a threaded opening which defines inlet 150 in the sidewall in the internal wall 32 as depicted in FIG. 1. It will thus beappreciated that upon the deenergization of field coil winding 152 thefluid is then free to pass through the inlet port 198 and into the rearchamber 24 until the compression spring 62 returns the piston 50 to theretracted position of FIG. 1.

The operation of hydraulic actuator will now be described with referenceto the control system depicted in block diagram form in FIG. 10. Thefeedback coil unit 60 is energized by an A-C source 200 and its output Vis one of the two input signals compared by an integrated circuitamplifier 202. The other input signal is received at terminal 204 from aremote condition sensitive element (not shown) and designated as V,. Theintegrated circuit amplifier 202 compares the control voltage V, withthe feedback voltage V and amplifies their difference to produce anoutput voltage V, proportional to the difference of the two inputvoltages. Schmitt trigger No. l senses the output voltage level V and ata preselected voltage level provides a positive gating of the switchdevice 206. The switch device 206 could, for example, use a Triacelement to provide this positive gating operation, which energizes motorcoil 76. The Schmitt trigger circuit No. 2, likewise, senses the outputvoltage V, and energizes the coil 152 of the dump valve assembly 140 ata preselected voltage level.

When the control voltage V exceeds the feedback voltage V by apreselected value, thedifference is derived by the integrated circuitamplifier 202 and ampli fied to provide a voltage level of sufficientmagnitude to switch on both Schmitt trigger circuits. Upon the switchingon of Schmitt trigger No. 1, the motor is energized. Simultaneously,Schmitt trigger No. 2 energizes coil 152 to attract armature 176 andthereby shut off the inlet port 198 of dumping valve 140. The piston 50and actuator element 44 are driven outwardly by the motor-pump assembly70 until the magnetic permeable portion 48 moves part way through thecenter of coil 60. The movement of magnetic permeable portion 48 throughthe center of coil 60 proportionally increases the efficiency of thefeedback transformer 60 and consequently its output voltage V increasesto approach the signal level of control voltage V When the controlelement 44 reaches a desired position, the output V, of the integratedcircuit amplifier 202 has decreased to less than the threshold voltagefor Schmitt trigger No. 1 so that the switch device 206 and associatedmotor coil 76 are deenergized. The output voltage V, at this time,however, has not decreased to the threshold voltage level for Schmitttrigger No. 2 so that dump valve remains closed. Consequently, thecontrol element 44 will be held in this stationary extended positionuntil the control voltage V drops sufficiently in value to reduce theintegrated circuit amplifier output voltage V to a value which willdeenergize Schmitt trigger No. 2 and permit the dump valve inlet 198 toopen. At this time, the hydraulic fluid bleeds out of the expandablechamber 53 permitting the actuator element to retract until the feedbackvoltage V F is again slightly less than the control voltage V,.. Thedump valve then recloses and holds the shaft position of actuatorelement 44 fixed until a further change in the control voltage V,occurs.

It will be understood that the details of the electronic control systemof FIG. 10 are not essential for an understanding of this invention. Fora complete description of the circuits shown in block diagram form andtheir operation, reference may be made to the copending U.S. Pat.application Ser. No. 326,854 which was filed on Jan. 26, 1973, and isassigned to the same assignee as this invention.

The electronic components for providing the electronic control system ofFIG. 10 may be assembled in the electronic componentscompartment 28above the forward chamber 26. A removable top cover 210 is provided overthe upper open end of the compartment 28 to permit access totheelectronic components mounted therein. Furthermore, one or moreauxillary switches 212 can be supported within the electronic componentscompartment 28 on a printed circuit board 214 such that their switchcontacts 216 are adapted to be actuated by one or more switch actuatormembers 220 as the control element 44 is extended or retracted. Each ofthe switch actuators 220 has a bead shape with a hollow center and ismounted on a switch actuator rod 222, which extends parallel to thecontrol element 44 and is connected to the control element 44 at itsouter end by means of a cross bar 224. The rear end of the switchactuator rod 222 is slidably received in a circular elongated recess 226formed lengthwise in the top wall 228 of the housing 22.

Referring to FIG. 6 there is shown a modified version of the actuatorelement illustrated with the embodiment of FIG. 1. The actuator elementof FIG. 6 differs from that shown in FIG. 1 in that the magneticpermeable portion 252 has a tapered nose segment 250 of magneticpermeable material. The magnetic permeable portion 250 is constructedfrom a sheet of magnetic into a cylindrical configuration and slippedover the reduced diameter portion 254. The axial displacement of theactuator element of FIG. 6 would be the same when used in the FIG. 1embodiment. By using a tapered nose segment of magnetic permeablematerial, the FIG. 6 actuator element changes the reluctance of thefeedback transformer coil 60 at a much slower rate as the actuatorelement slides through the coil. Other characterized strokes can beobtained by shaping the nose segment to have any desired curve or stepconfiguration to thereby vary the inductive feedback as desired.

There is illustrated in FIG. 7, a front perspective view of the actuatorelement described with reference to FIG. 1. By increasing the length offront cover plate 38 as shown in dotted lines 260 at the piston end ofthe actuator element, it is possible to increase the stroke of theactuator to any desired length. Similarly, by mounting a hollow end cap262 on the rear open end of actuator housing, it is possible to increasethe reservoir capacity of rear chamber and thereby increase its strokelength.

FIG. 8 illustrates one application of the actuator element 44. A toothedrack slide 264 is attached to the threaded end 265 of the actuatorelement 44 and cooperates with a rotatably mounted toothed gear 266 forreciprocally driving in a vertical direction a lever 268 which isrotatably mounted at one end 269. The intermediate portion of lever 268has a follower roller 272 disposed within the arcuate slot 273 of thedisk 276 fixed on the outer end of toothed gear 266.

FIG. 9 shows another application of the hydraulic actuator of thisinvention. In the FIG. 9 illustration, the actuator element 44 is shownas conjointly operating a radiator valve 270 and a control rod 284connected to a damper element (not shown). A mounting bracket 271 issupported on the front end of hydraulic actuator and has a connectingframe 275 on its upper end extending outwardly and interconnected to theradiator valve 270 in a conventional fashion. An actuator lever 276 ispivotally supported along its intermediate portion. on the upstandingboss 278 and is interconnected to actuator element 44 by a connectingpin 280 which extends crosswise through the actuator element 44 andactuator lever 276. The control element 44 projects outwardly through arectangular recess 282 formed in the actuator lever 276. The lower endof the actuator lever 276 is pivotally connected to a damper elementcontrol rod 284 by a connecting pin 286. Similarly, a connecting pin 288connects the top end of the actuator rod 276 to a control rod 290 thatoperates radiator valve 270.

In order to accommodate various tolerances and overtravel required instroking valve 270, an expandable and contractible coupler 292 is usedto interconnect control rod 290 and valve stem 294. Coupler 292 isconstructed of an elastomeric member 295 vulcanized between end plates296 and 297 which have threaded central openings for connecting to thethreaded end portions of central rod 290 and valve stem 294,respectively. By using coupler 292, it is possible to have overtravelfor both open and closed valve positions.

I claim:

1. A hydraulic actuator comprising an enclosed housing having a dividingwall separating the interior of said housing into first and secondchambers, and elongated actuator element extending in said secondchamber and slidingly supported through an opening in said housing forselective positioning at a plurality of discrete positions, a pistonsupported transversely on said actuator element and in slidable sealingengagement with the walls of said second chamber to thereby define anexpandable chamber with said dividing wall, biasing means for urgingsaid piston toward said dividing wall, pump means for introducing fluidunder pressure through a passageway formed through said dividing wallinto said expandable chamber, a separate dump valve means forselectively opening and closing a bleed passage formed through saiddividing wall into said expandable chamber for bleeding the fluid insaid expandable chamber when said bleed passage is opened to permit saidactuator element and associated piston to retract under the force ofsaid biasing means and to maintain said actuator element at any one ofsaid plurality of discrete positions when said bleed passage is closedand means for providing a variable control signal which continuouslyindicates any one of said plurality of discrete positions at which saidactuator element is located for use in the selective operation of saidpump means and the selective operation of said dump valve means.

2. A hydraulic actuator as defined in claim 1, wherein said pump meanscomprises a pump unit having an inlet in communication with said firstchamber and an outlet in communication with said passageway, and checkvalve means to prevent the flow of fluid from said expandable chamberinto said first chamber.

3. A hydraulic actuator as defined in claim 2, wherein said pump meansfurther comprises an electric motor for operating said pump unit,whereby the energization of said motor is effected by'said variablecontrol signal.

4. A hydraulic actuator as defined in claim 2, wherein said pump unitcomprises at least three plates stacked together in a side-by-sideabutting relation to define a pump housing, one of middle of said plateincluding a central recess having two pairs of opposing arcuateportions, a pair of pump gears in intermeshing relation rotatablymounted between the two end plates and nested within one of said pairsof opposing arcuate portions, and a pair of openings formed through oneof said end plates in alignment with said other pairs of opposingarcuate portions to define pump inlet and outlet.

5. A hydraulic actuator as defined in claim 1, wherein said positionsensing means comprises a transformer coil having an axial openingdisposed in the path of said actuator element and having a crosssectional area larger than the cross section of said actuator element,and wherein said actuator element comprises a nonmagnetic permeablelength portion and a magnetic permeable length portion.

6. A hydraulic actuator as defined in claim 5, wherein said magneticpermeable length portion includes a tapered nose segment.

7. A hydraulic actuator as defined in claim 1, wherein said dumpvalve'means comprises a core member having an axial opening disposed insaid first chamber, a valve fitting mounted in one end of said axialopening of said core member, a plunger slidably mounted inwardly of theother end of said axial opening, a ball valve disposed in said axialopening between said fitting and said plunger, an armature membercarried on said plunger outwardly of said axial opening, and a fieldcoil disposed about said core member and producing sufficient magneticforce when energized to attract said armature causing said plunger tomove against said ball valve with sufficient force to close the port insaid valve fitting.

8. A hydraulic actuator comprising an enclosed housing having a dividingwall separating the interior of said housing into first and secondchambers, an elongated actuator means including a magnetic permeablelength portion member extending in said second chamber and slidinglysupported through an opening in said housing for selective positioningat a plurality of discrete positions, a piston supported transversely onsaid actuator means and in slidable sealing engagement with the walls ofsaid second chamber to thereby define an ex pandable chamber with saiddividing wall, biasing means for urging said piston toward said dividingwall, pump means for introducing fluid under pressure through apassageway formed through said dividing wall into said expandablechamber, a separate dump valve means for selectively opening and closinga bleed passage formed through said dividing wall into said expandablechamber for bleeding the fluid in said expandable chamber when saidbleed passage is opened to permit said actuator means and said piston toretract under the force of said biasing means and alternatively tomaintain said actuator means at any one of said plurality of discretepositions when said bleed passage is closed, said actuator means beingextensible to a different one of said plurity of positions in responseto selective operation of said pump means and closing of said dump valvemeans, and means operative with said magnetic member for providing avariable control signal which continuously indicates any one of saidplurality of discrete positions at which said actuator means is locatedfor use in the selective operation of said pump means and the selectiveoperation of said dump valve means.

9. A hydraulic actuator as defined in claim 8, wherein said pump meanscomprises a pump unit having an inlet in communication with said firstchamber and an outlet in communication with said first passageway, andcheck valve means to prevent the flow of fluid from said expandablechamber into said first chamber.

10. A hydraulic actuator as defined in claim 9, wherein said pump meansfurther comprises an electric motor for driving said pump unit, wherebythe energization of said motor is effected by said variable controlsignal.

11. A hydraulic actuator as defined in claim 9, wherein said pump unitcomprises three plates stacked together in side-by-side abuttingrelation to define a pump housing, the middle of said plate including acentral cloverleaf-recess having two pairs of opposing arcuate portions,a pair of pump gears in intermeshing relation rotatably mounted betweenthe two end plates and nested within one of said pairs of opposingarcuate portions, and a pair of openings formed through one of said endplates in alignment with said other pairs of opposing arcuate portionsto define a pump inlet and outi let.

12. A hydraulic actuator as defined in claim 8 wherein said meanscomprises a transformer coil having an axial opening disposed in thepath of said actuator means and having a cross sectional area largerthan the cross section of said actuator means to accommodate themovement of, said actuator means relative to said transformer coil.

13. A hydraulic actuator as defined in claim 12, wherein said magneticpermeable length portion includes a tapered segment.

14. A hydraulic actuator as defined in claim 8, wherein said dump valvemeans comprises a core member having an axial opening disposed in saidfirst chamber, a valve fitting mounted in said second passageway, aplunger slidably mounted inwardly of the other end of said axialopening, a ball valve disposed in said axial opening between saidfitting and said plunger, an armature member carried on said plungeroutwardly of said axial opening, and a field coil disposed about saidcore member and producing sufficient magnetic force when energized toattract said armature causing said plunger to move against said ballvalve with sufficient force to close the port in said valve fitting.

15. A hydraulic actuator comprising an enclosed housing having adividing wall separating the interior of said housing into first andsecond chambers, an elongated actuator means including a magneticpermeable length portion extending in said second chamber and slidinglysupported through an opening in said housing for selective positioningat a plurality of discrete positions, a piston supported transversely ofsaid actuator means and in slidable sealing engagement with the walls ofsaid second chamber to thereby define an expandable chamber with saiddividing wall, biasing means for urging said piston toward said dividingwall, pump means for introducing fluid under pressure through apassageway formed through said dividing wall into said expandablechamber, 5 separate dump valve means for selectively opening and closinga bleed passage formed through said dividing wall into said expandablechamber for bleeding the fluid in said expandable chamber when saidbleed passage is opened to permit said actuator means and said piston toretract under the force of said biasing means and to maintain saidactuator means at any one of said plurality of discrete positions whensaid bleed passage is closed, and means operatively connected with saidpump means and said dump valve means including a transformer in saidsecond chamber for providing a variable control signal whichcontinuously indicates any one of said plurality of discrete positionsat which said actuator means is located in response to sliding movementof said magnetic member, both said pump means and said dump valve meanswhen energized effecting movement of said actuator means to at least oneof said plurality of discrete desired positions as well as effectingmovement of said magnetic member toward said transformer to provide acorrespondingly different value control signal for use in repositioningsaid actuator means, said actuator means being maintained at a desiredone of said plurality of discrete positions when said pump means isdeenergized and said dump valve means is energized, subsequentdeenergization of said dump valve means opening said bleed passage toeffect movement of said actuator means and said magnetic member by saidbiasing means toward another of said plurality of discrete positions andthe output of a correspondingly different signal by said transformerwith such movement.

16. A hydraulic actuator comprising a housing having at least a firstchamber, an expandable chamber, and a biased actuator means extending insaid first chamber for selective positioning at a plurality of discretepositions, pump means for introducing fluid under pressure into saidexpandable chamber to enable movement of said actuator means in a firstdirection including first input means for providing energizing power tosaid pump means, a separate dump valve means for selectively enablingfluid to be bled from said expandable chamber to permit movement of saidbiased actuator means in an opposite direction, and alternatively forselectively preventing bleeding of the fluid from said expandablechamber to maintain said biased actuator means at any one of saidplurality of discrete positions including second input means forproviding energizing posite directions,

1. A hydraulic actuator comprising an enclosed housing having a dividingwall separating the interior of said housing into first and secondchambers, and elongated actuator element extending in said secondchamber and slidingly supported through an opening in said housing forselective positioning at a plurality of discrete positions, a pistonsupported transversely on said actuator element and in slidable sealingengagement with the walls of said second chamber to thereby define anexpandable chamber with said dividing wall, biasing means for urgingsaid piston toward said dividing wall, pump means for introducing fluidunder pressure through a passageway formed through said dividing wallinto said expandable chamber, a separate dump valve means forselectively opening and closing a bleed passage formed through saiddividing wall into said expandable chamber for bleeding the fluid insaid expandable chamber when said bleed passage is opened to permit saidactuator element and associated piston to retract under the force ofsaid biasing means and to maintain said actuator element at any one ofsaid plurality of discrete positions when said bleed passage is closedand means for providing a variable control signal which continuouslyindicates any one of said plurality of discrete positions at which saidactuator element is located for use in the selective operation of saidpump means and the selective operation of said dump valve means.
 2. Ahydraulic actuator as defined in claim 1, wherein said pump meanscomprises a pump unit having an inlet in communication with said firstchamber and an outlet in communication with said passageway, and checkvalve means to prevent the flow of fluid from said expandable chamberinto said first chamber.
 3. A hydraulic actuator as defined in claim 2,wherein said pump means further comprises an electric motor foroperating said pump unit, whereby the energization of said motor iseffected by said variable control signal.
 4. A hydraulic actuator asdefined in claim 2, wherein said pump unit comprises at least threeplates stacked together in a side-by-side abutting relation to define apump housing, one of middle of said plate including a central recesshaving two pairs of opposing arcuate portions, a pair of pump gears inintermeshing relation rotatably mounted between the two end plates andnested within one of said pairs of opposing arcuate portions, and a pairof openings formed through one of said end plates in alignment with saidother pairs of opposing arcuate portions to define pump inlet andoutlet.
 5. A hydraulic actUator as defined in claim 1, wherein saidposition sensing means comprises a transformer coil having an axialopening disposed in the path of said actuator element and having a crosssectional area larger than the cross section of said actuator element,and wherein said actuator element comprises a non-magnetic permeablelength portion and a magnetic permeable length portion.
 6. A hydraulicactuator as defined in claim 5, wherein said magnetic permeable lengthportion includes a tapered nose segment.
 7. A hydraulic actuator asdefined in claim 1, wherein said dump valve means comprises a coremember having an axial opening disposed in said first chamber, a valvefitting mounted in one end of said axial opening of said core member, aplunger slidably mounted inwardly of the other end of said axialopening, a ball valve disposed in said axial opening between saidfitting and said plunger, an armature member carried on said plungeroutwardly of said axial opening, and a field coil disposed about saidcore member and producing sufficient magnetic force when energized toattract said armature causing said plunger to move against said ballvalve with sufficient force to close the port in said valve fitting. 8.A hydraulic actuator comprising an enclosed housing having a dividingwall separating the interior of said housing into first and secondchambers, an elongated actuator means including a magnetic permeablelength portion member extending in said second chamber and slidinglysupported through an opening in said housing for selective positioningat a plurality of discrete positions, a piston supported transversely onsaid actuator means and in slidable sealing engagement with the walls ofsaid second chamber to thereby define an expandable chamber with saiddividing wall, biasing means for urging said piston toward said dividingwall, pump means for introducing fluid under pressure through apassageway formed through said dividing wall into said expandablechamber, a separate dump valve means for selectively opening and closinga bleed passage formed through said dividing wall into said expandablechamber for bleeding the fluid in said expandable chamber when saidbleed passage is opened to permit said actuator means and said piston toretract under the force of said biasing means and alternatively tomaintain said actuator means at any one of said plurality of discretepositions when said bleed passage is closed, said actuator means beingextensible to a different one of said plurity of positions in responseto selective operation of said pump means and closing of said dump valvemeans, and means operative with said magnetic member for providing avariable control signal which continuously indicates any one of saidplurality of discrete positions at which said actuator means is locatedfor use in the selective operation of said pump means and the selectiveoperation of said dump valve means.
 9. A hydraulic actuator as definedin claim 8, wherein said pump means comprises a pump unit having aninlet in communication with said first chamber and an outlet incommunication with said first passageway, and check valve means toprevent the flow of fluid from said expandable chamber into said firstchamber.
 10. A hydraulic actuator as defined in claim 9, wherein saidpump means further comprises an electric motor for driving said pumpunit, whereby the energization of said motor is effected by saidvariable control signal.
 11. A hydraulic actuator as defined in claim 9,wherein said pump unit comprises three plates stacked together inside-by-side abutting relation to define a pump housing, the middle ofsaid plate including a central cloverleaf-recess having two pairs ofopposing arcuate portions, a pair of pump gears in intermeshing relationrotatably mounted between the two end plates and nested within one ofsaid pairs of opposing arcuate portions, and a pair of openings formedthrough one of said end plates in alignment with said other pairs ofopposing arcuate portions to define a pump inlet and outlet.
 12. Ahydraulic actuator as defined in claim 8 wherein said means comprises atransformer coil having an axial opening disposed in the path of saidactuator means and having a cross sectional area larger than the crosssection of said actuator means to accommodate the movement of saidactuator means relative to said transformer coil.
 13. A hydraulicactuator as defined in claim 12, wherein said magnetic permeable lengthportion includes a tapered segment.
 14. A hydraulic actuator as definedin claim 8, wherein said dump valve means comprises a core member havingan axial opening disposed in said first chamber, a valve fitting mountedin said second passageway, a plunger slidably mounted inwardly of theother end of said axial opening, a ball valve disposed in said axialopening between said fitting and said plunger, an armature membercarried on said plunger outwardly of said axial opening, and a fieldcoil disposed about said core member and producing sufficient magneticforce when energized to attract said armature causing said plunger tomove against said ball valve with sufficient force to close the port insaid valve fitting.
 15. A hydraulic actuator comprising an enclosedhousing having a dividing wall separating the interior of said housinginto first and second chambers, an elongated actuator means including amagnetic permeable length portion extending in said second chamber andslidingly supported through an opening in said housing for selectivepositioning at a plurality of discrete positions, a piston supportedtransversely of said actuator means and in slidable sealing engagementwith the walls of said second chamber to thereby define an expandablechamber with said dividing wall, biasing means for urging said pistontoward said dividing wall, pump means for introducing fluid underpressure through a passageway formed through said dividing wall intosaid expandable chamber, s separate dump valve means for selectivelyopening and closing a bleed passage formed through said dividing wallinto said expandable chamber for bleeding the fluid in said expandablechamber when said bleed passage is opened to permit said actuator meansand said piston to retract under the force of said biasing means and tomaintain said actuator means at any one of said plurality of discretepositions when said bleed passage is closed, and means operativelyconnected with said pump means and said dump valve means including atransformer in said second chamber for providing a variable controlsignal which continuously indicates any one of said plurality ofdiscrete positions at which said actuator means is located in responseto sliding movement of said magnetic member, both said pump means andsaid dump valve means when energized effecting movement of said actuatormeans to at least one of said plurality of discrete desired positions aswell as effecting movement of said magnetic member toward saidtransformer to provide a correspondingly different value control signalfor use in repositioning said actuator means, said actuator means beingmaintained at a desired one of said plurality of discrete positions whensaid pump means is deenergized and said dump valve means is energized,subsequent deenergization of said dump valve means opening said bleedpassage to effect movement of said actuator means and said magneticmember by said biasing means toward another of said plurality ofdiscrete positions and the output of a correspondingly different signalby said transformer with such movement.
 16. A hydraulic actuatorcomprising a housing having at least a first chamber, an expandablechamber, and a biased actuator means extending in said first chamber forselective positioning at a plurality of discrete positions, pump meansfor introducing fluid under pressure into said expandable chamber toenable movement of said actuator means in a first direction includingfirst input means for providing energizing power to said pump means, aseparate dump valve means for selectively enabling fluid to be bled fromsaid expandable chamber to permit movement of said biased actuator meansin an opposite direction, and alternatively for selectively preventingbleeding of the fluid from said expandable chamber to maintain saidbiased actuator means at any one of said plurality of discrete positionsincluding second input means for providing energizing power to said dumpvalve means, said biased actuator means being movable in said firstdirection towards a different one of said plurality of discretepositions in response to simultaneous energization of said pump meansand said dump valve means, said actuator means being held at saiddifferent position with deenergization of said pump means, and means forproviding a variable control signal which continuously indicates any oneof said plurality of discrete positions at which said biased actuatormeans is located in response to movement of said biased actuator meansin either of said first or opposite directions.